I'm embarrassed to ask this about scaling faces
First make sure to start with a 1 base: scale the base cylinder to one in all dimensions.

If you've added a standard cylinder, it should be 2x2x2 meters.

So scale it 0.5 in object mode, then apply scale CtrlA to have the same in edit mode.

Once done enter edit mode, take the faces you need (maybe the bottom of the cylinder) and scale 12.9: S12.9

Now select the top part and scale 12.7: S12.7.

The proportions should be good. Now scale all 0.001 to have it in millimeters.



In the blend file above, top part is 12.7 and bottom part is 12.9.